4) Lady Deathstrike
We last saw Lady Deathstrike in X2, but the events of that movie were wiped from the timeline by X-Men: Days of Future Past. X-Men: Apocalypse proved that as a result of that, characters can be born again in different time periods (just try not to think too much about it), and with the cybernetically enhanced Reavers set to play a role in Logan, her making a surprise appearance here could definitely work.
Assuming that we’re going to spend some time with Laura Kinney while she’s under the care of Weapon X, perhaps we could meet Deathstrike as a child, too. Even if she just makes what would essentially be a background appearance, Fox could go some way in setting the stage for her to return somewhere down the line, possibly as the enemy of X-23 assuming Deathstrike fails to escape alongside what could be her childhood friend.